Characteristics of zinc oxide based multi-layer bulk acoustic wave devices

Piezoelectric thin films have broad applications in transducers, resonators, and filters. Thin film bulk acoustic wave (BAW) resonators and filters have many advantages, such as small size, low insertion loss and lower power consumption than their bulk crystal based ancestors. In particular, piezoelectric zinc oxide (ZnO) thin films have been used for multi-layer surface acoustic wave and BAW devices due to their high electromechanical coupling coefficients and ease of deposition.; This dissertation focuses on the research and development of thin film resonators based on ZnO and the new piezoelectric material magnesium zinc oxide (MgxZn1-xO). Simulation tools based on the modified Christoffel-Bechmann Method and Ballato's model are developed to analyze the piezoelectric properties of MgxZn1-xO based multilayer structures.; It was determined that in MgxZn1-xO/n+ZnO/r-Al 2O3 based thin film resonators, where a n+ ZnO layer is used as the bottom electrode, multi-mode coupling occurs at the substrate/film interface. In this structure, the c-axis of MgxZn1-x O film is in the surface plane, resulting in a shear wave excitation when thickness excitation is used. This shear wave couples with quasi-shear and quasi-longitudinal waves in r-Al2O3, leading to excitation of a longitudinal wave in the MgxZn1-xO and ZnO layers. This modified multi-mode transmission line model is more accurate than the previously reported single-mode model. A novel method for tailoring the acoustic properties of thin film resonators is developed by adjusting the Mg composition in MgxZn1-xO or by using Mg xZn1-xO/ZnO multilayer structures. This allows further flexibility in design of MgxZn1-xO based thin film resonators, in addition to varying the piezoelectric layer thickness. This tailoring property is demonstrated experimentally through a novel shear wave multi-mode thin film resonator using MgxZn1-xO/ZnO heterostructures on r-Al2O3, grown by metal organic chemical vapor deposition. Tailored single longitudinal mode resonators are also demonstrated using MgxZn1-xO thin films sputter deposited on Si, isolated from the substrate with a SiO2/W Bragg reflector acoustic mirror.; This work will find important applications in high performance thin film resonator filters for telecommunications, as well as in shear wave resonator chemical/biochemical sensors operating in liquid environments.